In a conventional diesel engine, reverse rotation likely occurs at the start thereof. For example, with respect to a manually started diesel engine having a single cylinder, a flywheel is rotated while fuel is injected into the diesel engine under decompression. The decompression is canceled after the rotation speed has risen up. At this time, a large quantity of fuel having been injected during the decompression is gasified and activated according to increase of pressure and temperature, so as to start ignition before the piston reaches its top dead point. Consequently, the piston, even assisted by the inertial rotation of the flywheel, cannot reach the top dead point and is backed by the ignition, whereby the reverse rotation of the diesel engine occurs.
The reverse rotation causes functional exchange between the intake system and the exhaust system, such that air is inhaled into a muffler, and exhausted from an air cleaner. The problem arises that the exhaust gas damages components of the intake system. Therefore, a reverse rotation preventing mechanism is provided on a camshaft for opening and closing an intake or exhaust valve, as disclosed in Japanese Laid-Open Gazette No. Hei 6-146938.
In the reverse rotation preventing mechanism disclosed in the document, a decompression member provided on a camshaft is pressed against an exhaust cam or an intake cam by a spring, so that the decompression member is rotatable following the rotation of the exhaust or intake cam by the frictional pressure. However, the reverse rotation preventing mechanism, requiring additional components such as the decompression member and the spring, causes increase of parts and costs.